Growth and stress tolerance of tomato seedlings (Lycopersicon esculentum Mill.) in response to seed treatment with paclobutrazol

'Marglobe' tomato seeds were soaked (24 h at 22degreesC) or matric primed (4.0 MPa for 7 d at 22degrees C) in 50 mgl(-1) of paclobutrazol (PB), a triazole growth retardant. Plants from these treatments and from non-treated seeds or seedlings whose shoots were sprayed with 10 mg PB P at 14 days after planting (DAP) were assessed for growth and physiological responses before and after factorially combined stresses of drought (a 14 d period of drying cycles until severe wilting) or heat (4 h at 50degrees C) were initiated at 24 DAP in the greenhouse. At 23 DAP, shoot heights and shoot and root dry weights of plants from PB-soaked seeds or PB-sprayed plants were similar, but lower than those from nontreated or PB-primed seeds. This pre-stress PB-mediated growth suppression was associated with greater stress tolerance as revealed by greater increases in shoot and root dry weights during the 10 d post-stress recovery period. Further support for the increased stress tolerance of plants from PB-soaked seeds or PB-sprayed plants than occurred with plants from non-treated seeds or PB-primed seeds was their higher leaf xylem pressure potential, lower leaf electrolyte leakage, and lower loss of leaf total chlorophyll during the 10 d post-stress recovery period.